The Research Of Test Method For Electromagnetic Shielding Effectiveness Of Screened Twisted Cable

With the development of digital communication and electronic equipment, the frequency range of screened twisted cable used for network cabling is raised, so its electromagnetic shielding performance becomes more important. The parameters which describe electromagnetic shielding performance of screened twisted cable include:transfer impedance, screening attenuation, unbalance attenuation and coupling attenuation. At present, the international electrotechnical commission has published many standard, such as IEC62153-4-3, IEC62153-4-5 and IEC61156-1 show the detailed introduction and rules for the characterization parameters and the related test method. However, the upper limitting frequency can only reach 1GHz according to standard method. This article achieves the measurement by combining mixed-mode S-parameters theory with standard method, which improves test efficiency and broadens frequency range. The main research work is as follows:1) Design and production of impedance matching network and terminating fixture. This article adopts the direct calculation method according to the standard IEC62153-4-3 to design the impedance matching network. In order to achieve terminating better, this article designs a PCB for connecting each wire of the cable under test. In addition, all parts are connected by thread, which is beneficial to install and debug.2) Evaluation of the performance of balun. Balun is a very important device in the test system, that relates to the reliability of the test result. Lossless infinite-inductance centre-tapped coils are necessary to test the performance of balun, but the coils are difficult to acquire. This article evaluates the performance of balun by use of mixed-mode S-parameter to simplify the testing process.3) Test by combining mixed-mode S-parameter theory with standard method. By contrast with the results of unbalance attenuation based on mixed-mode S-parameter method and standard method, this paper concludes that measuring unbalance attenuation based on mixed-mode S-parameter is available and the frequency range is improved to 3GHz. This article measures coupling attenuation by combining mixed-mode S-parameter theory with absorbing clamp method range from 30MHz to 1GHz. The test results show measuring coupling attenuation based on mixed-mode S-parameter is available and the test efficiency is improved. This article achieves the measurement of coupling attenuation by combining the mixed-mode S-parameter with mixing chamber method to contrast the results of absorbing clamp method range from 500MHz to 1GHz. The results based on two methods are matched well and the test frequency range of coupling attenuation is improved to 3GHz.